The present invention relates to a positive electrode of a sodium-sulfur storage battery, in which metal sulfide layer is disposed between positive electroconductive material and a solid electrolyte tube for increasing utilization factor of positive reactant.
Conventionally, as shown in FIG. 1 which is a longitudinally sectional view, a sodium-sulfur storage battery is provided with a positive chamber formed around a solid electrolyte tube 1 made from beta-dash-alumina, beta-alumina or the like which is permeable to sodium ion. An alpha-alumina ring 2 is jointed with glass to the upper end of the tube 1, and a positive cover 3 is thermocompressively joined to the lower surface of the alpha-alumina ring 2. Positive electroconductive material 6 such as graphite felt or carbon felt in which positive reactant 5 such as sulfur or sodium polysulfide is impregnated is disposed between the solid electrolyte tube 1 and a battery housing 4 welded to the positive cover 3. A circular plate 7 of graphite felt is disposed at the bottom, and a bottom cover 8 is welded to the battery housing 4. At a battery operation temperature from 300.degree. C. to 370.degree. C., when the battery is discharged, sodium of negative reactant is ionized, passes through the solid electrolyte tube 1 and reacts on the positive reactant 5 in the positive chamber to form discharge product Na.sub.2 Sx. When the composition of the positive reactant 5 in the positive electroconductive material 6 changes to Na.sub.2 S.sub.2.7 to Na.sub.2 S.sub.3.0, the discharge stops. When charged, the sodium in the discharge product returns to the negative chamber. As shown in FIG. 2, at prompt discharge, all sulfur becomes discharge uncomposition, and theoretical capacity value is recognized. However, at one cycle charge, only about 70% of the reactant is utilized for the charge. Further, at the charge-discharge cycles thereafter, it shows similar tendency, and the charging volume at 150 cycles becomes 54% of the theoretical capacity value and the utilization factor of the positive reactant becomes about 54%. It is also recognized in tests of another batteries that the charging value and the utilization factor further decrease after above cycles.
Accordingly, it is an object of the invention to provide an improved sodium-sulfur storage battery, in which utilization factor of positive reactant is improved.
The essence of the present invention is to dispose metal sulfide layer between positive electroconductive material and a solid electrolyte tube for increasing utilization factor of positive reactant.